Re: hardware for a server - Mailing list pgsql-general

A B wrote:
>> Don't put SAS drives on a 3ware controller.  They say that works now, but
>> they haven't really gotten it right yet--their controllers are still only
>> good with SATA drives.
>>
>
> How bad will it be with SAS drives? Is there so little performance
> gain witn 3ware+SAS?
>

The concern isn't performance.  I did a SAS+3Ware install recently and
was struck by how the SAS supported seemed bolted out without being
fully integrated.  My concerns are more related to whether they've
really handled all the possible drive failure cases, given that they are
missing basics like http://www.3ware.com/KB/article.aspx?id=15456

> Scott Marlowe stated in earlier reply that Seagates ES.2 disks are not
> very good, which would leave the SAS Cheetah discs with the LSI card.
> The LSI card is down to 128 MB memory.
> But LSI+SAS is still the clear winner over 3ware with  512 MB and SAS/SATA?
>

I think it will be too close to predict which will work better for your
application, and that you'd be better off thinking in terms of your
storage and monitoring needs instead of stressing over the possible
performance difference between these two options.  Those are the two
reasonable paths here, and I don't believe they lead to such
dramatically different places at the end from a performance perspective
that speed should be the only thing factoring into how to make that
decision now.

>> Don't mix SAS and SATA; vendors will tell you it works, but it's extremely painful when it doesn't, and that happens
sometimes.
>>
>
> Does that also forbid the case when you create two raid arrays, let
> say a raid-1 with only SATA discs (huge discs) and a  raid-10 with
> only SAS drives? (as your example with the 2/6 split)
> There are internal SATA controllers so I don't have to bother the Raid
> card with a pair of SATA drives, but I'd prefer to use the BBU  for
> all the drives.

The situation I never have satisfying results with involves mixing SAS
and SATA drives on the same controller; I assumed you'd be using them in
separate RAID arrays, which doesn't change that opinion.

Write caches typically work only against drives connected directly to
that controller.  You could easily split the OS drive out onto your
internal SATA controllers.  However, I think you'll be disappointed with
the results, because software RAID-1 for the boot drive in particular is
more difficult to manage and recover from failures with.

--
Greg Smith  2ndQuadrant US  Baltimore, MD
PostgreSQL Training, Services and Support
greg@2ndQuadrant.com   www.2ndQuadrant.us